Magnetic devices are used in many applications, such as in magnetic stimulation devices, speakers and so forth. Magnetic devices tend to generate heat because of resistive losses in the coil(s) that generate magnetic fields(s), and the amount of heat generated is proportional to the amount of power consumed by the device. Thus, high-voltage magnetic devices that consume large amounts of power, such as those used in magnetic stimulation therapy, can become very hot when in operation. The environment in which the magnetic device operates—or the operating characteristics of the device itself—may dictate that the device operate under a certain temperature threshold. For example, in magnetic stimulation therapy, the temperature of a magnetic stimulation device used to generate a therapeutic magnetic field should be kept below approximately 41.5° C. to stay within certain regulatory requirements (e.g., FDA guidelines). If a magnetic stimulation device is to be operated at temperatures exceeding 41.5° C., such regulatory requirements dictate that the device manufacturer and/or health practitioner must meet additional guidelines to prove that operation of the device is safe. These additional requirements increase complexity of operation and overall cost, and are best avoided when possible.
Conventionally, a magnetic stimulation device used for such therapy is used until it reaches a threshold temperature, and then the therapy is temporarily halted until the stimulation device cools. Such an arrangement therefore adds to the time required to perform a treatment, which is undesirable for both the patient and the health practitioner. Alternatively, a second magnetic stimulation device may need to be used (i.e., swapped with the first device when the first device reaches the threshold temperature) so as to continue the therapy without interruption while the first, overheated stimulation device cools. This arrangement is also undesirable because of the added expense associated with the purchase and maintenance of an additional magnetic stimulation device. Furthermore, additional time is required of the patient and health practitioner, as the second magnetic device will need to be set-up and/or calibrated to perform magnetic stimulation therapy on the patient. Because the set-up and/or calibration steps provide opportunities for operator error, requiring the operator to perform such steps multiple times may decrease the overall safety level of the treatment.
Conventional cooling solutions typically involve the use of air or fluid cooling mechanisms. An air cooling mechanism may involve a fan that rapidly circulates cooled or room temperature air past the magnetic device. A fluid cooling mechanism may involve the circulation of a cool fluid past the magnetic device, where the fluid cools the device and is heated in the process, and then to a cooling mechanism, after which the fluid is returned to the magnetic device. Both mechanisms have several drawbacks. For example, both mechanisms require additional moving parts (e.g., fans, cooling mechanisms such as a refrigeration or heat exchange unit, etc.), which add to the cost and complexity of the magnetic device. Furthermore, the additional moving parts add to the potential for a device malfunction.
An additional consideration of magnetic devices is acoustical noise generated by the magnetic coil of a magnetic device as the coil is energized. For example, when the coil is energized, it creates a strong magnetic field that, in many applications, rapidly changes in intensity. The changing magnetic field causes windings of the coil to experience hoop stresses that intermittently stress the windings, which causes a sharp acoustic click.
Such noise is especially pronounced in magnetic stimulation devices, as the therapeutic magnetic fields are created by pulsing the stimulation device's coil. Such noise is problematic for patients, as the stimulation device is typically located in close proximity to the patient's head, and therefore the noise from the stimulation device may be uncomfortable. In addition, a health practitioner who is repeatedly exposed to such noise may be adversely affected. A conventional solution, placing earplugs in the patient's ears, is undesirable because it is an additional step to perform in the therapeutic process and does not solve the problem of the noise caused by the device in the treatment facility (e.g., physician's office, hospital, etc.). In addition, the use of earplugs is undesirable because some psychiatric or young patients may be uncooperative, and therefore the use of earplugs unnecessarily complicates the procedure.
Thus, a conventional solution for the reduction of acoustical noise is the placement of noise reduction material around all or part of a magnetic device. Alternatively, a chamber containing a partial vacuum may be formed around the magnetic device, because a partial vacuum contains very few particles that may propagate a mechanical (sound) wave. However, such noise reduction techniques have the disadvantage of adversely affecting heat transfer for cooling. For example, the best noise reduction materials are fabricated to contain air pockets that do not transfer noise well. However, such air pockets also have the characteristic being poor conductors of heat. The same is true to an even greater extent in the case of a vacuum. Thus, if such a noise reduction technique is used, the magnetic stimulation device cannot be adequately cooled. Attempting to mitigate such a dilemma by placing acoustical material, or forming a partial vacuum, around a cooling system that is itself arranged around a magnetic device is undesirable because of the added size, cost and complexity of the resulting device.
Conventionally, ferrofluids have been used to cool audio speaker systems, which is a lower voltage application when compared to a magnetic stimulation device or other high voltage magnetic device. A ferrofluid is a fluid with suspended ferromagnetic particles. The ferromagnetic particles can be influenced by the magnetic field created by the speaker so as to enhance fluid convection between the speaker and a heat sink to cool the speaker. An additional benefit of ferrofluids is that they can be used to cool a device while still performing noise reduction, because a ferrofluid typically does not support shear waves. Furthermore, a mismatch in sound velocity may also cause the reflection of some of the sound waves.
Unfortunately, even the ferrofluid solution used in connection with speakers has disadvantages that may render it unsuitable for use with high voltage magnetic devices, such as a magnetic stimulation device. For example, the ferrofluid used in connection with speaker cooling, while a dielectric when exposed to normal speaker-level voltages, may be unable to maintain dielectric isolation at the higher voltage levels used in connection with a magnetic stimulation device. As a result, arcing or other problems may occur.
Therefore, what is needed is a ferrofluidic cooling apparatus, system and method for high voltage applications. More particularly, what is needed is an apparatus, system and method for convectively circulating a ferrofluid to cool a high voltage magnetic device. Even more particularly, what is needed is an apparatus, system and method of using a ferrofluid to cool such a high voltage magnetic device while also mitigating acoustical noise.